1. Field of the Invention
The present invention relates to the field of image processing and in particular to tone-mapping of luminance levels of an original image to adjusted luminance levels for improved appearance of the image on an output medium.
2. Description of the Related Art
In the field of image processing and in the art of photography, tonal range, also known as luminance range or brightness range, is the difference between the maximum (highlights) and minimum (shadows) amounts of light reflected from a subject. With respect to color images, tonal range refers to the lightness of the colors, without regard to their hue or colorfulness. Although due to the Helmholtz-Kohlrausch effect, colors of higher chroma appear lighter than measurements would indicate. The differences in tonal value create contrast. Contrast, in turn, defines shapes and helps objects to be seen more clearly in a viewed image.
In the art of photography, techniques have been developed in attempts to appropriately map luminance levels of the original captured scene to luminance levels for display of the image on an output medium. One technique is the use of a global illumination algorithm, which computes luminance values for an image, which closely match the radiance values of the corresponding real world scene. The resulting image from such an algorithm is usually displayed on an output medium such as a CRT or photographic paper. This step requires a conversion from luminance values of the captured image to corresponding luminance values that are within a range that can be displayed by the output medium.
The above technique is complicated by the fact that the range of luminance values that can be displayed by a typical output medium is much smaller than the range that exists in the real world. Whereas scene luminance levels can easily vary by a factor of 10,000, luminance levels for CRT displays only vary by a factor of 100, and print luminance levels only vary by a factor of about 20. In addition, it is not necessarily desirable to accurately match the original scene luminance levels to a corresponding level for display on the output medium. As various studies have shown, indoor viewers, for example, usually prefer images that have a higher contrast level compared to that of the original scene. Furthermore, viewers often prefer that certain objects of interest within the image appear within a certain tone range to achieve an optimized tone appearance. For example, in an image of a person sitting in an office, the person's face would likely be of more interest to a typical viewer than other objects appearing within the image.
One technique, known as the “zone system”, was developed for use by photographers in taking and developing photographic images. The zone system starts with obtaining measurements of the tone response curves of photographic paper on which the image is to be displayed. According to the zone system, the tonal range of a photographic print is divided into zones 0 to X. Zone 0 is full black; zone X is full white. Although some shadings of a viewed image can be distinguished in zones I and IX, any part of the image in which texture detail is to be distinguished should generally fall between zones II and VIII.
Under the zone system, the effects of exposure and development times are quantified in order for the photographer to understand the effects of these parameters on the ultimate tonal reproduction of objects within the output image. Aperture and exposure time are then set by the photographer based on these parameters. In this manner, the above method works backwards from the envisioned placement of tonal values on the final image on the output medium. For example, the photographer must consider, at the time of capturing the image, the development of the photographic print on the output medium, the development of the negative, and the parameters for capturing the original image by the camera. A major disadvantage of the zone system is its requirement for performance of complex calculations by the photographer at the time of taking the picture. Clearly, the application of the zone system is manual and requires expertise well beyond the skills of an amateur.
Another approach, proposed by Jack Holm and described in U.S. Pat. No. 6,249,315, concerns an attempt to apply the zone system to digital photography. In general, the Holm approach maps tonal values within each of the various tonal zones to new tonal values based on characteristics of an entire image. This approach includes construction of an image dependent non-linearity model by constructing a scaled image based on the original image and then deriving image statistics from the scaled image. Image statistics from the scaled image are derived by using preliminary information about the capture device which captured the original image in order to produce scaled image focal plane data, determining significant statistical values of the scaled image focal plane data, and determining flare characteristics of the original image based on viewing environment of the original image.
The Holm approach suffers from the same complexity disadvantage suffered by the zone system, namely, that of requiring extensive knowledge about the camera and the output medium. Furthermore, the Holm approach requires extensive knowledge about the conditions under which the image was captured, in order to calculate the flare values. Another drawback of the Holm approach is that it achieves tonal mapping based on tonal statistics of the entire image without consideration of individual areas within the image that a typical viewer would likely find most interesting. This is because the algorithm developed by Holm has no way of inferring which parts of the captured image are more interesting and which parts are less interesting to a viewer. Therefore, the Holm approach assumes predetermined tone-mapping from the original image to new tonal zones, regardless of which objects in the image are of interest to the viewer.
The current generation of digital cameras typically use a same tonal mapping for all images captured, regardless of the types of objects contained within the image that may be of interest to a viewer. Accordingly, resulting printed images obtained from such cameras are not optimal in providing aesthetic pleasure to the viewer because the tone-mapping has not been adjusted based on objects of interest in the image.